Direct generation of chirp-free solitons without external compression in normal-dispersion fiber lasers is a long-term challenge in ultrafast optics. We demonstrate near-chirp-free solitons with distinct spectral sidebands in normal- dispersion hybrid-structure fiber lasers containing a few meters of polarization-maintaining fiber. The bandwidth and duration of the typical mode-locked pulse are 0.74 nm and 1.95 ps, respectively, giving the time-bandwidth product of 0.41 and confirming the near-chirp-free property. Numerical results and theoretical analyses fully reproduce and interpret the experimental observations, and show that the fiber birefringence, normal-dispersion, and nonlinear effect follow a phase-matching principle, enabling the formation of the near-chirp-free soliton. Specifically, the phase- matching effect confines the spectrum broadened by self-phase modulation and the saturable absorption effect slims the pulse stretched by normal dispersion. Such pulse is termed as birefringence-managed soliton because its two orthogonal-polarized components propagate in an unsymmetrical"X"manner inside the polarization-maintaining fiber, partially compensating the group delay difference induced by the chromatic dispersion and resulting in the self-consistent evolution. The property and formation mechanism of birefringence-managed soliton fundamentally differ from other types of pulses in mode-locked fiber lasers, which will open new research branches in laser physics, soliton mathematics, and their related applications.

Dong Mao;Zhiwen He;Yusong Zhang;Yueqing Du;Chao Zeng;Ling Yun;Zhichao Luo;Tijian Li;Zhipei Sun;Jianlin Zhao

Key Laboratory of Light Field Manipulation and Information Acquisition,Ministry of Industry and Information Technology,School of Physical Science and Technology,Northwestern Polytechnical University,710129 Xi'an,China;College of Electronic and Optical Engineering&College of Microelectronics,Nanjing University of Posts and Telecommunications,210046 Nanjing,China;Guangdong Provincial Key Laboratory of Nanophotonic Functional Materials and Devices&Guangzhou Key Laboratory for Special Fiber Photonic Devices and Applications,South China Normal University,510006 Guangzhou,China;Department of Electronics and Nanoengineering and QTF Centre of Excellence,Aalto University,Aalto,Finland

光：科学与应用（英文版）

[1]Dong Mao;Zhiwen He;Yusong Zhang;Yueqing Du;Chao Zeng;Ling Yun;Zhichao Luo;Tijian Li;Zhipei Sun;Jianlin Zhao-.Phase-matching-induced near-chirp-free solitons in normal-dispersion fiber lasers)[J].光：科学与应用（英文版）,2022(02):230-241

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